1. Field of the Invention
The present invention relates to a mechanical element, in more detail, a crank connected to a pedal of a bicycle or a crank connected to a handle rotating rotary body, and a bicycle using the crank.
2. Description of the Related Art
One end of a crank rotating a rotary body is fixed to the shaft of a rotating body and the other end is connected to a member applying force to a handle and a pedal etc. Torque T rotating the rotary body is calculated by multiplying the length of the crank which corresponds to an arm length, that is, (by) the magnitude of force applied to the end of the crank. This is expressed by the following Formula.T=F×L, where F is force and L is arm length.
As can be seen from the Formula, it is possible to acquire larger torque from the same magnitude of force by increasing the length of the crank. When this is applied to a bicycle, since larger torque is achieved from the same force, it is possible to increase the speed of the bicycle and more easily climb a steep slope.
However, although it is possible to achieve larger torque by increasing the length of the crank, the rotational radius correspondingly increases and the arm or leg rotating the rotary body deviates from the normal motion range, which makes the rotation of the crank difficult and causes fatigue and discomfort.
The crank may be, for example, a bicycle crank connected with a pedal of a bicycle or a crank connected with a handle of a rotary body that is manually rotated. In the condition of use of the cranks, force is applied only in a predetermine rotation section in the entire rotation section where the crank rotates. For example, for the bicycle crank, torque is achieved by pressing the pedal with feet only while the pedal moves down from above in the entire rotation section.
In this case, large torque can be achieved if the length of the crank increases in the section where force is applied to rotate the rotary body, while the length of the crank decreases in the section where force is not applied, that is, in an idle section, such that if the rotational radius decreases, the arm or leg does not deviate from the normal motion range and the rotation may become easy.
An example of achieving larger torque by changing the length of the crank has been disclosed in Korean Patent Registration No. 10-0400648-0000 (Application No: 10-2001-0011635, Filing date: 2001 Mar. 7, Publication No. 10-2002-0071553, Publication Date: 2002 Sep. 13), titled “Device for Rotation of a Crank Type with a Variation Length”, in order to achieve the technical object described above. The patent that has proposed a device for rotation of a crank type with a variation length not only is complicated, but has a large number of parts, such that the manufacturing cost increase and it may difficult to be mounted on the existing bicycles.
Further, the bicycles with a gearshift that are used to achieve larger torque uses several gears to more easily climb a hill, in which although the torque increases and it is possible to climb a hill with ease, it is required to reduce the speed by shifting in order to achieve large torque, such that the speed is necessarily reduced.